ABSTRACT The long-term goal of this proposal is to elucidate the molecular mechanism controlling mammalian foregut and early liver development. The largest exocrine gland in the body the liver produces bile and is the primary site for detoxification. It also performs important endocrine functions by secreting homeostatic blood proteins and regulating glucose levels through glycogen storage. A recent trans-NIH report Action Plan for Liver Disease Research (2004) recognized that a better understanding of embryonic liver development would provide important insights into human liver disease and promote our ability to harness embryonic stem cells as a renewable source of tissue for transplantation. The mouse embryonic liver is induced from the ventral foregut endoderm by Fgf signals from the cardiac mesoderm. While we increasingly understand the genetic pathways regulating proliferation and differentiation of hepatoblasts after the liver bud has formed, the earlier events linking endoderm patterning to hepatic specification are less clear. In Xenopus we recently determined that differential Wnt/eta-catenin signaling regulates endoderm fates. Our data supports a model where during gastrula and early somite stages secreted Wnt- antagonists in the anterior endoderm establish foregut identity and initiate a molecular cascade leading to liver development. In contrast, the posterior endoderm has high eta-catenin activity, due to Wnt ligands secreted from the lateral/axial mesoderm, which represses foregut fate and promotes intestinal development. We propose to test this hypothesis using mouse genetics and embryonic explants to characterize the underlying molecular mechanism. Moreover, we will investigate whether analogous pathways are important for liver development in humans using endoderm cultures derived from human embryonic stem cells. The results of this proposal will directly impact efforts to generate therapeutically useful endoderm tissue for the treatment of liver disease in humans. Aim 1. Determine if repression of eta-catenin activity in the anterior endoderm is required for foregut and liver development using mouse genetics. Aim 2. Define when eta-catenin needs to be repressed and examine how the temporally distinct Wnt and Fgf pathways interact during hepatic development, using mouse embryonic explant cultures. Aim 3. Investigate the role of Wnt signaling in promoting human foregut and liver lineages from HESCs